This invention relates to methods for texturing glass substrates used in conjunction with magnetic disks.
One method for manufacturing a magnetic disk comprises the steps of texturing a glass substrate, depositing a magnetic film on the glass substrate, depositing a protective layer on the magnetic film, and applying a lubricant to the protective layer. One reason for texturing the substrate is to reduce the friction between the magnetic disk and a read/write head used in conjunction with the disk. The above-incorporated U.S. patent application Ser. No. 07/105,612 U.S. Pat. No. 4,833,001 discloses a method for texturing a glass substrate using chemical etching. During chemical etching, the glass substrate is subjected to either a gaseous or aqueous etching medium comprising HF.
FIG. 9 is a SEM photograph of a portion of a glass surface etched by a gaseous etching medium. Porous reaction products having a thickness almost equal to the etching depth form on the surface which consist of fluorides and contain very little silicon. It is believed that the major component of the glass (the silicon atoms) reacts with the etchant and forms gaseous SiF.sub.4, which is released from blowholes which dot the etched glass surface.
We theorize that during HF etching, a porous chemical compound barrier (e.g., material 1 in FIG. 1, which consists of a skeletal or sponge-like compound) and blowholes 6 form on the surface of glass substrate 2. Barrier material 1 acts as a permeable mask during chemical etching. As a result, as etching proceeds, mountains 3 are formed beneath material 1 (see FIG. 2) and valleys form below blowholes 6. Material 1 is removed after etching by rinsing substrate 2 with water, thereby leaving the profile illustrated in FIG. 3.
At the conclusion of the etching process, mountains 3 generally have a height less than or equal to about 700.ANG. at a reference length of 250 .mu.m and greater than or equal to about 50.ANG. at a reference length of 50 .mu.m. (The term "reference length" is also known as a "standard length", which is defined in Japanese Industrial Standard B 0601-1982 "Definitions and Designations of Surface Roughness", incorporated herein by reference, and in the '612 patent application.) The mountain pitches are between 0.1 and 50 .mu.m.
We have discovered that if prior to etching, glass substrate 2 includes a microcrack (also known as a latent crack in the glass industry) or scratch, after subjecting the disk to an HF chemical etchant, the disk surface will include excessively high mountains around the microcrack or scratch such as the mountains illustrated in the SEM photograph of FIG. 4, which is observed with a 75.degree. tilt angle. These mountains (hereinafter referred to as "linkage bumps") have a height greater than the height of the other mountains on the substrate surface and can cause the resulting magnetic disk to fail a glide height test. In addition, if a read/write head used in conjunction with the resulting magnetic disk repeatedly strikes a linkage bump during head flight or contact start-stop operations, a head crash can result. Also, the presence of linkage bumps affects the static friction coefficient (.mu.s) exhibited by the resulting magnetic disks.
FIG. 10 is a SEM photograph of the section of a glass surface with a scratch which was etched by a gaseous etching medium. Thick reaction products (e.g. masking material 1) are not formed over the scratch.
We theorize that during chemical etching, masking material 5 (which is different in characteristics from that of masking material 1) tends to accumulate in the vicinity of microcracks such as microcrack 4 (see FIG. 5a). Masking material 5 masks the glass from the etchant more efficiently than masking material 1 elsewhere on the disk surface, thereby forming linkage bump 7.
Sufficiently high mountains 3 (FIG. 3) can be attained under certain etching conditions without forming chemical compound barriers which result in formation of high linkage bumps. However, these etching conditions tend to decrease the surface density of mountains 3. Accordingly, it would be desirable to eliminate the microcracks or scratches from the disk to prevent linkage bump formation.